Triploid grass carp (Ctenopharyngodon idella) are sterile fish used to control nuisance aquatic vegetation. This species provides long-term aquatic plant control, e.g., nominally five to seven and a maximum possible of ten years, at less cost than either herbicides or mechanical harvesting. One limitation on their use is establishing an optimum stocking level, i.e., too few and the aquatic vegetation is not affected and too many and it is totally eliminated. A second limitation is associated with the latter in that they have a propensity to leave stocking sites, thus impacting desirable vegetation in other locations.
Recently, scientists achieved stocking densities in small impoundments that allowed a targeted level of aquatic plant control. However, achieving desired levels of control in large reservoir systems has been inconsistent. Nevertheless, techniques to age and collect triploid grass carp in large reservoir systems were developed. These techniques allowed scientists to project population trends. The ability to stock and predict subsequent populations of triploid grass carp facilitates achieving optimal populations.
Any carp that migrate may impact native aquatic vegetation in floodplain lakes and in estuaries. In these areas in particular, aquatic plants are needed to keep the natural balance of the environment. Aquatic plants are the primary source of oxygen production in many standing waters as well as providing a protective haven for small fish. In estuaries, these carp may consume native aquatic vegetation required for controlling erosion or needed as nursery habitat for economically important fisheries.
Given the need to optimize population density at stocking sites and to limit potentially damaging effects outside the stocking sites, one control mechanism is ability to limit the life span of these carp. For example, larger fish require more food, thus as they get larger, fewer are needed to maintain the same control of undesirable vegetation. This capability allows resource managers to precisely control population mix and achieve consistency in aquatic plant control in large reservoir systems. Further, this capability limits the time that escaped migrating carp roam freely. Hence, implementing life span control via a passive “plant and forget” method using bioerodible materials for timing delivery of toxins in implants permits introducing triploid grass carp as a suitable management tool for controlling undesirable aquatic vegetation in large impoundments. The term “bioerodible” includes self-destruction of an implant by dissolution, biodegradation, or both.